Research On The Preparative Technique And Sensing Characteristics Of Thin Core Long-period Fiber Gratings Fabricated By CO₂ Laser

With the characteristics of transmission-type band-rejection, low back-reflections, low insertion loss, wide bandwidth, sensitive to ambient environment and easily integrated, Long-Period Fiber Grating?LPFG? has been researched and developed in optical fiber communication and optical fiber sensing. In recent years, LPFG fabrication with CO₂ laser has gradually become a hot spot. CO₂ laser writing has the advantages of simple preparation process, flexible adjustment of parameters, strong controllability, high efficiency and low cost. Combining with different characteristics of specialty fibers, CO₂ laser writing is able to produce a specialty long period fiber grating which emerges different features based on traditional advantages of long period fiber grating, which shows its great application prospect in the field of optical fiber. This paper introduced Thin Core Fiber?TCF? in which inscribed LPFG and Ultra-Long-Period Fiber Grating?ULPFG?,ULPFG based on CO₂ laser, research on the related theory and did some experimental studies on preparation technology and sensing characteristics, then got some innovative research results. The main content included:?1?The formation mechanism of inscribing LPFG based on CO₂ laser was analyzed from the following four aspects, residual stress relaxation, glass densification, dopant diffusion and physical deformation. In the process of refractive index change caused by CO₂ laser writing, both the laser power, irradiation time, irradiation area of the CO₂ laser, the imposed axial force and the material of the optical fiber itself should be considered. The coupled-mode theory and its transmission matrix were introduced, the coupled-mode equation and coefficient was deduced, and then the spectral characteristic of LPFG was qualitatively analyzed. At last, the coupled-mode theory and equation of ULPFG were introduced.?2?A LPFG platform based on CO₂ laser was set up, including the selection of CO₂ laser, the installation of optical component and adjustment of the optical path. The detail of the preparation process was introduced from the angle of light path, and each parameter was explained. The effect of each parameter was analyzed from three aspects, which were the power of the CO₂ laser, the scanning cycles and the external force. Then the Thin Core Long Period Fiber Grating?TC-LPFG? and Thin Core Utral-Long Period Fiber Grating?TC-ULPFG? with excellent performance were made by summarizing the rules and optimizing the parameters.?3?The sensing characteristics of the TC-LPFG was studied. The sensing experiment of curvature was completed. The characteristic of discernable bending direction of the LPFG made by CO₂ laser for single side scanning of the TCF was analyzed by introducing an equivalent method. The cascaded structure based on Mach-Zehnder Interferometer was designed and multi-parameters simultaneous measurement scheme was implemented by using different loss peaks produced by superposition of transmission and interference spectrum. The measured curvature sensitivity was 28.82dB/m-1 through the way of intensity demodulation and the measured temperature sensitivity was 67.2pm/? through the way of wavelength demodulation. The cascaded structure can not only measure multi-parameters simultaneously, but also effectively avoid the cross sensitivity.?4?The sensing characteristics of the TC-ULPFG was studied. The spectrum of the TC-ULPFG was analyzed, and the sensing experiment of the curvature and temperature was completed. The spectrum of the TC-ULPFG presented a number of loss peaks which were corresponding to the different orders of the cladding mode with different diffraction orders. The multi-parameters simultaneous measurement scheme of single optical fiber device was realized by using the different sensitivities of those loss peaks for the external parameters. The appearance of cross sensitivity in the measurement process was effectively avoided through introduced the method of wavelength and intensity demodulation simultaneously, the curvature sensitivity is 97.77dB/m-1; the temperature sensitivity is 62.94 pm/?.